The invention relates to an active current transformer of the type having a primary winding conveying current to be measured 1; with W.sub.1 turns and a secondary winding conveying measuring current I.sub.2 with W.sub.2 turns and a detector winding coupled with the secondary winding whereby means of a variable gain amplifier, the induced voltage of the detector winding creates a current in the secondary winding, the magnetic flux of which compensates the magnetic flux permeating the detector winding and produces large current transforming ratio.